1. Field of the Invention
The present invention relates generally to a system for controlling data write operations in data storage devices, and more particularly to a system for preventing writing data off track.
2. Description of Related Art
In disk drives a position error signal (PES) generated from head output signals is typically used by a servo to control the position of the head which in turn determines where data is written to and read from. Position error signals can be determined from the amplitude of a burst signal obtained by reading a servo pattern on the disk. As an example, the method disclosed in published Unexamined Japanese Patent Application No. 6-243617 for determining a position error signal from a burst signal is described below.
FIGS. 5A-5D illustrate a conventional method for determining a position error signal from a burst signal. Four types of servo patterns A to D are recorded at a plurality of places on a magnetic disk at constant intervals. A bidirectional arrow R shows a radial direction and the servo patterns A to D show the peripheries of data tracks Nxe2x88x921 to N+2. As shown in FIG. 5A, each of the servo patterns A to D comprise a square having a width almost equal to the width of a disk track. Servo track B is recorded so as to be offset by one data track width from servo track A. Similarly servo track D is offset one data track from servo track C. Servo tracks B and C are offset from each other by one half of a data track width. The servo tracks A to D are successively read by a magnetic head in the circumferential direction of a magnetic disk, that is, the direction of the arrow C.
In hard disk drives a magnetic head reads servo patterns as it is moved over the disk in a radial direction. The servo patterns A and B and the servo patterns C and D are respectively considered as pairs. Moreover, it is assumed that signals obtained when a magnetic head reads servo patterns are SA to SD. Furthermore, by computing a position error signal MP=SAxe2x88x92SB)/SA+SB) and a position error signal NP=(SCxe2x88x92SD)/(SC+SD), the position error signal MP is changed as shown by the continuous line in FIG. 5B and the position error signal NP is changed as shown by the continuous line in FIG. 5C.
The MP position error signal changes linearly when passing through the vicinity of the central portion of each concentric data track. Similarly, the NP position error signal changes linearly when passing through the vicinity of the boundary of each data track. Moreover, both the MP and NP position error signals respectively have a period in which their value is constant. This is caused by the fact that the longitudinal dimension of the read portion (gap) of a magnetic head is smaller than the width of a data track. This period is referred to as a dead zone because the position error signal cannot be used to indicate the position of a magnetic head in this period.
By switching between an MP position error signal and an NP position error signal in accordance with the position of a magnetic head on the data track and capturing only the linear portions of the signals, it is possible to generate a position error signal that is linear over a wide range of positions of the magnetic head as shown in FIG. 5D. Because the position error signal, PES, shown in FIG. 5D linearly corresponds to the position of a magnetic head on a magnetic disk, it is possible to determine the position of the magnetic head based on the position error signal.
Even if the position of a magnetic head is controlled in accordance with the above PES in a magnetic disk drive, various write instabilities may cause errors when recording data. In hard disk drives using magnetoresistive (MR) heads, write instability conditions can cause the change of a PES to be small when the head deviates from the correct track, i.e. when the head is off track. FIG. 6 shows an example in which the change of a PES is small when the head is off track, that is, the slope of position error signals MR and NP becomes small in the linear portion. In this case, even if a position error signal PES is obtained by switching between the position error signals MR and NP, there are still discontinuities. Therefore, although a magnetic head may actually be located at an off-track position, the PES may not indicate that the magnetic head is off track and the magnetic head will then write data in the off-track position. This can cause a read failure when data in the next track is attempted to be read. When this type of error occurs in a manufacturing test environment, it can cause the drive to be rejected as defective.
It is an object of the present invention to solve the above problems and provide a system for preventing data write operations when the magnetic head is off track.
The present invention provides a system for preventing off track data write operations in data storage devices such as hard disk drives. In a preferred embodiment, the amplitude of a head output signal is monitored. The output signal amplitude value is determined by measuring the voltage of a capacitor used in a Variable Gain Amplifier (VGA) in an Automatic Gain Control (AGC) circuit of a hard disk drive channel unit.
The measured head output signal amplitude is compared with a previously determined minimum normal head output signal amplitude reference value to determine whether the head output signal amplitude is within a normal operating range. If the measured value is less than the reference value then the head is in an off track position, and the data write operation is inhibited.
A further feature of a preferred embodiment of the present invention is to use radial position dependent amplitude reference values. For example, a disk can be divided into groups of one or more adjacent tracks, and a head output signal reference value can be generated for each group of tracks.
The present invention detects and prevents off-track write operations which otherwise may not be detected by position error signal (PES) based methods. In the present invention a write operation is determined to be off-track and therefore is prevented, when a head output signal amplitude is smaller than a reference value. Such an off track condition could go undetected in a conventional PES based system when the change of a PES is small due to a write instability condition. The present invention thereby improves the reliability of data write operations.